Currently, Advent runs with 2 leaders, myself and Kae Lee. Communication between us is very high and so far it is unheard of, for us, not to come to an agreed conclusion. Just in case we cannot come to an agreement, however, our Charter provides for an 'Advisor': someone who we can approach in regards to issues that we both feel very strongly about. That Advisor is picked from the Ministerial leadership (of our currently small alliance) for a period of two months, in order to also provide an avenue to free up the 'log jam' mentality.
Really, I believe that the real issue behind all leadership, regardless of size, is communication. If no one talks and discusses things honestly with each other, then you are going to have problems. People might resent a decision, not because they weren't heard out on the issue, but because they didn't voice their true feelings. It is no ones fault, except for that person who didn't say what they felt, but the resentment will still linger. To a certain degree, I suppose, it is a bit about trust as well. Being able to freely speak without fear of recourse, or hurting feelings, is paramount to constructive conversation.
Just as a quick aside - even when you run an alliance where decisions are made at the very top - that doesn't mean that you don't listen to your Ministers, Deputy Ministers, or even the Membership at large. Engaging people, and offering them a chance to voice their opinion makes them feel a part of the organization - and it also helps to formulate better policy in my opinion. With all things, however, there is a cut off point. A point where the discussion either repeats itself, or time has run out on set goals - but that is why leaders are leaders, and good ones use the resources at hand, as much as they can, before making a decision.
So what I am basically saying is, you can have an even number of leaders, if you have the necessary alternatives to break deadlocks (which, in our case, is basically an appointed third leader). Also, if you find the right person to be a leader of an alliance with, and you communicate properly and openly, then you might not ever need the third vote.